The invention relates to the field of the optical recording of information on a medium, such as an optical disc.
The information is in principle stored on the medium in the form of physical marks that are singularities of controlled dimensions that provide an optical contrast enabling them to be read by a laser beam detection system.
The physical marks may be impressions formed by moulding of a polycarbonate substrate (for example for a DVD-ROM device)—they are then recorded once and for all. They may also be formed by zones recorded in sensitive layers through the action of a write light beam—the recording may then be reversible (possible erasure or even re-recording) or may be irreversible (no possible erasure or rewriting).
When seeking to increase the density of information recorded on an optical disc, the limitation is in general the performance of the information read device. The basic principle is that physical information written in the disc cannot be read if their size is smaller in size than the resolution limit of the optical system that will be used to read this information. Typically, with reading using a red laser of 650 nm wavelength and a numerical aperture of 0.6, it cannot normally be hoped for information smaller in size than 0.3 microns to be correctly read.
However, methods referred to as super-resolution methods have been devised for reading information having a physical size smaller than the optical resolution limit (LR=(λ/4)·NA) where λ is the resolution and NA the numerical aperture of the focussing optic of the laser. These methods are based on the non-linear optical properties of certain materials. The “non-linear properties” is understood to mean that certain optical properties of the material change with the intensity of the light that they receive. The read laser itself will locally modify the optical properties of the material through thermal, optical, thermooptical and/or optoelectronic effects over smaller lengths than the size of the read laser spot. Because of the change in property, information present in this very small volume becomes detectable, whereas it would not be detectable without this change.
The phenomenon exploited is based mainly on two properties of the read laser that will be used:                firstly, the laser is focused very strongly so as to have an extremely small section (of the order of the wavelength), but the power distribution of which is gaussian, being very strong at the centre but highly attenuated on the periphery; and        secondly, a read laser power is chosen such that the power density over a small portion of the section, at the centre of the beam, significantly modifies an optical property of the layer, whereas the power density outside this small portion of the section does not significantly modify this optical property, the optical property being modified in a direction aimed at reading information that would not be able to be read without this modification.        
For example, in the case of super-resolution discs, the reflectivity is locally increased over a zone smaller than the diameter of the laser beam. It is this modification due to the non-linear optical properties that will allow smaller marks, which are not normally detectable, to be read.
In a prior patent application, filed in France under the number FR 07/00938 on 9 Feb. 2007, (publication FR 2912539), an optical storage structure operating in super-resolution mode was proposed. This structure comprises a substrate (preferably made of polycarbonate) provided with physical marks, the geometric configuration of which defines the recorded information, a superposition of three layers above the marks of the substrate, and a transparent protective layer above this superposition, the superposition comprising an indium antimonide or gallium antimonide layer inserted between two dielectric layers of a zinc sulphide-silicon oxide (ZnS—SiO2) compound.
This structure is favourable because it requires a relatively low read laser power to read the information in super-resolution mode with a satisfactory signal/noise ratio. Now, the question of the read power is critical since, on the one hand, a high enough power is necessary to obtain a super-resolution effect by locally changing the optical properties, but, on the other hand, too high a power has a tendency for the recorded information to be gradually destroyed, limiting the number of possible read cycles, whereas it is desirable to have as large a number of read cycles as possible.
By carrying out trials on these structures based on InSb or GaSb between two ZnS—SiO2 layers, it has however been found that the choice of read power is not simple, in that super-resolution readout is not possible with too low a power, while excessively high power is unnecessary or threatens the preservation of the information or even of the optical medium, and it seems that there is an intermediate power zone, below the optimum power that allows super-resolution readout, for which the stored information is irremediably degraded by the read laser.
This observation was made based on repeated measurements on specimens having uniformly distributed marks recorded in super-resolution.
It is therefore desirable to provide an optical information read system having means for optimizing the read laser power while taking into account this risk of irreversible degradation of the information for intermediate power levels below this optimum power. Furthermore, it is desirable for these means themselves not to involve a read power that lies in the degradation risk zone.